1. Field of the Invention
The present invention relates to an intermediate frequency circuit in a television tuner.
2. Description of the Prior Art
As shown in FIG. 6, an intermediate frequency circuit in a conventional television tuner, which circuit in the conventional television tuner will hereinafter be referred to simply as xe2x80x9cintermediate frequency circuit,xe2x80x9d has an intermediate frequency tuning circuit (xe2x80x9cIF tuning circuitxe2x80x9d hereinafter) 22 which tunes to an intermediate frequency signal, IF, outputted from a mixer 21, a trap circuit 23 which attenuates a audio IF carrier in a lower adjacent channel, and intermediate frequency amplifier (xe2x80x9cIF amplifierxe2x80x9d hereinafter) 24.
A television signal, RF, in a reception channel selected by a tuning circuit (not shown) is inputted at balance to the mixer 21, in which it is mixed with a local oscillation signal inputted at balance from a local oscillator 25 and is frequency-converted to the intermediate frequency signal, IF. The intermediate frequency signal, IF, is outputted at balance from the mixer 21 and is passed through the IF tuning circuit 22, whereby signals adjacent to both sides of the intermediate frequency signal, IF, are attenuated. The trap circuit 23, which is constituted of a parallel resonance circuit, is disposed between the IF tuning circuit 22 and the IF amplifier 24. The trap circuit 23 is inserted in series into one signal line. The resonance frequency (trap frequency) of the trap circuit 23 is made coincident with the frequency (60.25 MHz according to the television specification in Japan) of an adjacent audio carrier in an intermediate frequency band.
FIG. 7 shows an intermediate frequency circuit using an integrated circuit component 26 including a mixer 21, an IF amplifier 24 and a local oscillator 25 which are formed within a single package. The integrated circuit component 26 is provided with two output terminals 26a and 26b of the mixer 21 and two input terminals 26c, 26d and two output terminals 26e, 26f of the IF amplifier 24. An intermediate frequency signal, IF, outputted from the mixer 21 is inputted to the two input terminals 26c and 26d in the integrated circuit component 26 via an IF tuning circuit 22 and a trap circuit 23 which are disposed outside the integrated circuit component 26. The intermediate frequency signal, after amplified by the IF amplifier 24, is outputted at balance from the output terminals 26e and 26f. 
The conventional intermediate frequency circuit thus configured has a frequency characteristic (response) in an intermediate frequency band such that, as shown in FIG. 8, a video IF carrier, P, (say 58.75 MHz in Japanese channel) and a audio IF carrier, (likewise, 54.25 MHz) are at almost the same level and that the level of a audio IF carrier (Sxe2x88x921) in an adjacent channel attenuates 8 dB or so with respect to a peak level. The attenuation value depends on the quality factor Q of the trap circuit 23 itself and a circuit impedance such as an input impedance of the IF amplifier 24. The higher the value of Q of the trap circuit 23 itself and the lower the circuit impedance, the larger the attenuation value. However, taking into account the balance between Q of the trap circuit 23 which is obtained in a practical range and the circuit impedance, there is determined such a degree of attenuation value as shown in FIG. 8.
At the attenuation value obtained in the above conventional intermediate frequency circuit the disturbance by the audio IF carrier in the adjacent channel is not negligible, thus requiring a still larger attenuation value. However, a limit is encountered in making Q of the trap circuit 23 itself high. Lowering the circuit impedance may be a solution, but lowering the circuit impedance gives rise to the problem that the loss in the intermediate frequency band increases. Further, since the trap circuit is inserted in series into a signal line, the integrated circuit component including the mixer 21 and the IF amplifier 24 is required to be provided with input terminals of the IF amplifier, with consequent increase in cost of the integrated circuit component.
The present invention has been accomplished in view of the above-mentioned problems and it is an object of the invention to provide an intermediate frequency circuit in a television tuner capable of enlarging the attenuation value of a audio IF carrier in an adjacent channel without increasing the loss in an intermediate frequency band.
It is another object of the present invention to permit the use of an integrated circuit component including a mixer and an IF amplifier and not provided with input terminals for the IF amplifier, in case of constituting an intermediate frequency circuit by using the integrated circuit component.
According to the present invention, for achieving the above-mentioned objects, there is provided an intermediate frequency circuit in a television tuner, comprising a mixer which outputs an intermediate frequency signal, an intermediate frequency amplifier for amplifying the intermediate frequency signal outputted from the mixer, and a feedback circuit disposed between an input end and an output end of the intermediate frequency amplifier, wherein a audio IF carrier of an adjacent intermediate frequency signal adjacent to the intermediate frequency signal is fed back from the output end to the input end by the intermediate frequency amplifier by the feedback circuit.
Preferably, the feedback circuit is constituted of a series resonance circuit connected between the input and output ends of the intermediate frequency amplifier, and a resonance frequency of the series resonance circuit is made coincident with the frequency of the audio IF carrier.
Preferably, a parallel resonance circuit is connected between the input end of the intermediate frequency amplifier and the ground, a coupling coil is inductively coupled to a resonance coil in the parallel resonance circuit and is connected at one end thereof to the output end of the intermediate frequency amplifier, an amplified intermediate frequency signal is taken out from an opposite end of the coupling coil, the feedback circuit is constituted of both the parallel resonance circuit and the coupling coil, and a resonance frequency of the parallel resonance circuit is made coincident with the frequency of the audio IF carrier.
Preferably, a low-pass filter is provided at the output end of the intermediate frequency amplifier, and a coil of the low-pass filter also serves as the coupling coil.